The present invention generally relates to filter technology and, more specifically, to filtering units having flat filter members, such as membrane filters, depth filters or prefilters used in laboratory or research facilities.
A wide variety of filtering apparatus exists for separating particles from liquid. The choice of which type of filtering apparatus to use in a given application depends mainly on particle size and throughput parameters. Generally, particles of a size greater than 10 xcexcm may be separated from liquids using a macrofiltration process, typically involving the use of depth filters. The depth filters used in macrofiltration may be made of paper, asbestos, plastic or glass fiber. Macrofiltration is frequently used in series with microfiltration to remove larger particles prior to the microfiltration process. This type of multi-stage filtering process can increase the rate at which liquid can be passed through a membrane filter or, in other words, increase the throughput rate. In microfiltration, particles greater than about 200 Angstroms but less than about 100 xcexcm are separated from a liquid typically using a membrane filter as a final filter.
Conventional membrane filters are much thinner and more fragile than depth filters and are typically available in a wide variety of particle retention capabilities and in a variety of materials, shapes and sizes. Membrane filter material may be, for example, nylon, polyethersulfone, PTFE or cellulose acetate. Membrane filters are available generally as either thin discs or in a folded cartridge configuration. Cartridge filters are usually recommended for more quickly filtering large volumes of liquid, such as volumes greater than about 400 liters. These operations may be carried out with the cartridge filter attached in-line and with the liquid flowing from one end of the cartridge to the other.
Often, laboratory or research facility needs may be met using disc-shaped membrane filters having a diameter of, for example, 50 mm to 75 mm. These facilities may not require the throughput rate provided by a cartridge filter, but high throughput is still desirable in order to carry out the filtering procedure as quickly as possible. Larger diameter membrane filters provide a larger throughput rate due to their increased surface area, however, the increase in throughput rate comes at a much greater incremental cost. Therefore, the larger diameter membrane filter units can be undesirable from this standpoint. Also, these types of filtering units typically come preassembled with the disc-shaped membrane filter permanently affixed between upper and lower containers or, alternatively, in an unassembled condition. In this latter case, the user must handle the fragile, disc-shaped membrane filter during assembly. Each of these options has disadvantages in that the pre-assembled filtering unit does not allow versatility in the selection of filter size or pore size following purchase by the user. On the other hand, requiring the user to handle the delicate filter member during assembly may result in damage or contamination. Finally, increasing the throughput rate by prefiltering the liquid is not easily accomplished as it generally necessitates the use of multiple filtering units.
To address various problems in this area, such as the problems mentioned above, it would be desirable to provide a filtering unit having an easily assembled and replaceable filter, such as a membrane filter, and a more efficient and less costly manner of providing a greater throughput rate while retaining the ability to use smaller diameter filters.
The present invention therefore provides a filtering unit including a first fluid receiving member, such as an upper container, for receiving fluid to be filtered and a second fluid receiving member, such as a lower container, for receiving the fluid from the upper container after the filtration process has taken place. In accordance with the invention, a filter cassette including a support member connected with a flat filter member is configured for disposition between the fluid to be filtered in the upper container and the lower container. The filter cassette is separate and may be removable with respect to each of the upper and lower containers. The cassette is capable of forming a selectively engageable seal between the fluid in the upper container and the lower container to ensure that fluid in the upper container passes through the filter member before passing into the lower container. Preferably, a threaded connection is formed between the upper and lower containers. This allows the lower container to be separated from the upper container after a filtering operation and thereby allows the contents to be stored or used in further processing. In a preferred embodiment, the flat filter member may include a membrane filter and/or a prefilter or depth filter. These filter members may be made from conventional materials as discussed above. The use of both of these types of filter members can allow a multi-stage filtering operation to be performed with a single filtering unit.
Specifically, for multi-stage filtering processes, a plurality of the separately attachable filter cassettes may be connected in a serial fashion between the upper and lower containers. As an option, a single filter cassette could be constructed with two different filter members, such as a prefilter or depth filter and a membrane filter connected in series. As still another option, one filter member may be permanently affixed to the filtering unit while another may be part of the inventive filter cassette. These types of multi-stage filtering unit may be used to increase throughput volume in an easy, efficient and relatively low cost manner.
Also in accordance with the invention, the support member of the filter cassette preferably includes a peripheral seal for engaging an inside wall surface of the upper container. This peripheral seal may be a flexible outer edge portion of the support member or may be a soft, compliant seal member or portion, such as an O-ring member, connected either integrally with the support member or in a removable manner with respect to the support member. The lower end of the upper container preferably includes a stepped-in portion for centering and engaging the peripheral sealing member in a liquid-tight manner during a filtering operation. This unique configuration, coupled with the use of a peripheral, selectively engageable sealing member allows for easy and consistent sealing engagement to be accomplished by the user between the upper container and the filter cassette. For utilization of the invention in typical laboratory and research settings, the lower container communicates with a port for attaching a source of vacuum. This port may be part of the lower container or, more preferably, part of a coupling between the upper and lower containers. Vacuum introduced through the port helps pull liquid through the filter member or members and into the lower container while the filter member or members trap particles larger than the filter pore size.
The invention further contemplates the provision of a separate filter cassette, as generally described above, for use in a filtering unit having upper and lower containers. A significant advantage of the invention is that the user may purchase the containers, with or without a permanently affixed filter, and purchase the filter cassettes separately. The user may selectively choose the appropriate filter cassette at the time of use according to the filtering needs. In the past, as noted above, this versatility required the user to assemble a filter unit by handling the delicate filter member itself or stock entire preassembled units including the upper and lower containers and the permanently affixed filter member. The present invention eliminates these problems and provides a filter cassette that is much easier to handle without damage or contamination and is more cost efficient to stock.
Methods of filtering are further encompassed by the present invention and are generally defined by the use of filtering units and/or cassettes falling within the scope of the invention. More particularly, these filtering methods can generally include connecting a filter cassette comprising a support member affixed to a flat filter member between fluid receiving members, such as upper and lower containers, with the support member held in sealing but removable engagement with respect to both the upper and lower containers. The filter member thereby provides a liquid flow path from the upper container to the lower container. The method further includes introducing a liquid containing particles into the upper container and onto the filter cassette, and passing the liquid through the filter member and into the lower container to retain the particles in and/or on the filter cassette.
The method can further, and more particularly, comprise connecting the filter cassette within the upper container such as by providing a peripheral seal on the support member and engaging that seal frictionally with an inner wall surface of the upper container in accordance with the inventive principles discussed above. The step of passing liquid through the filter member preferably includes introducing a vacuum into the lower container to actively draw liquid through the filter member. Also in accordance with the inventive concepts, more than one filter cassette may be connected between the upper and lower containers and, to facilitate a multi-stage filtering process, one of the filter members may be a prefilter, while the other filter member may be a final filter. It will be understood that, while one of the filter members will be part of a filter cassette constructed in accordance with the invention, the other filter may be permanently affixed between the liquid in the upper container and the lower container. It will be further appreciated that additional modifications or variances using the inventive concepts disclosed herein are also possible within the scope of this invention.
These and other advantages and objectives of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the presently preferred embodiments of the invention taken in conjunction with the accompanying drawings.